1. Field of the Invention
The present invention relates generally to sealed rotary rock bits and, more particularly, to a means to pressurize the lubrication systems utilized in such bits.
2. Description of the Prior Art
A rotary rock bit generally consists of a main bit body adapted to be connected to a rotary drillstring. A conventional bit usually includes two or more legs integrally connected to form a bit body. Each leg includes a cutter, rotatably mounted on a journal pin, extending from the leg. Bearings are provided between the cutter and the journal pin to promote rotation of the cutter and means are provided on the outer surface of the cutter for disintegrating the formation as the bit and cutter rotate.
In lubricated rock bits, a typical lubrication system is provided which includes an annular seal located at or near the backface of the cutter cone to prevent the lubricant from leaking from the bearing area to the exterior of the rock bit and to prevent drilling fluid and debris from entering the bearing area. The lubrication system further includes a reservoir filled with a lubricant, typically a high viscosity petroleum grease, with passages provided to communicate the reservoir with the bearing space between the cutter cone and the journal pin.
Compensators are utilized within the prior art that function to equalize the pressure on the "mud" side of the seal with the pressure on its lubricant side under varying pressure conditions. U.S. Pat. No. 3,476,195 describes a low pressure relief valve which functions to relieve excessive gas pressures in the bit during normal drilling. However, during the raising and lowering of the drill bit into an existing borehole, a substantial pressure differential can develop. Providing a low pressure relief system would not operate properly because the relief valve would open prematurely before deleterious pressure differentials are reached. Therefore, too much lubricant would be lost during the drill bit lowering operation.
U.S. Pat. No. 4,161,223, assigned to the same assignee as the present invention, teaches the use of a pressure relief valve for sealed bearing rock bits. A sealed bearing rotary cone rock bit is disclosed in which a combined manual venting and pressure relief system is located within the lubrication reservoir of the rock bit. The rock bit comprises a main bit body having a plurality of legs extending downwardly therefrom. Each leg has a rolling cone cutter rotatively mounted thereon. A lubrication system is provided in each leg to provide lubricant to the bearing area between the cutter and the leg. The lubrication system includes a lubricant reservoir chamber that communicates via passageways with the bearing area. The reservoir further includes a rubber boot molded around a metal stiffener sleeve. A cover cap is attached to the rubber boot. The rubber boot is in the form of a resilient membrane and is exposed through the cover cap to the exterior of the rock bit and through the passageways to the interior of the lubrication bearing area. The vent and pressure relief system comprises an annular seat formed in the wall of the reservoir. A valve face is formed on the rubber boot and is biased against the annular seat by means of a belleville spring acting on the cover cap. If excessive pressure develops within the lubricant reservoir, the excess is blown off through the valve seat. Excessive internal pressures can also be manually vented safely by a slight prying action on the cover cap without removal of the cover cap.
The system just described is a relatively high pressure system which compensates for internal and external pressures. However, where there is zero pressure differential during operation of the bit, there is still a danger of detritus entering through the dynamic seal face formed between the cone and the journal to the internal lubricant passages. Any water or debris within the bearing surfaces formed between the journal pin and the rotary cones will quickly cause catastrophic destruction of the bit.
A means is provided within the reservoir cavity to pressurize the viscous grease within the seal lubrication system. By pressurizing the internal lubrication system, detritus or debris is prevented from passing by the seal between the cutter cone and the journal pin due to the increased pressure within the lubrication system. Thus, a positive differential pressure is created across the dynamic seal between the bearing pin and the bearings of the cutter cone.
Therefore, the present invention is an improvement over the prior art in that the internal lubricant passages are pressurized to prevent detritus or mud from entering the bearing surfaces during operation of the bit.